Biomimetic vascular network and devices using the same

ABSTRACT

The invention provides method of fabricating a scaffold comprising a fluidic network, including the steps of: (a) generating an initial vascular layer for enclosing the chamber and providing fluid to the cells, the initial vascular layer having a network of channels for fluid; (b) translating the initial vascular layer into a model for fluid dynamics analysis; (c) analyzing the initial vascular layer based on desired parameters selected from the group consisting of a characteristic of a specific fluid, an input pressure, an output pressure, an overall flow rate and combinations thereof to determine sheer stress and velocity within the network of channels; (d) measuring the sheer stress and the velocity and comparing the obtained values to predetermined values; (e) determining if either of the shear stress or the velocity are greater than or less than the predetermined values, and (f) optionally modifying the initial vascular layer and repeating steps (b)-(e). The invention also provides compositions comprising a vascular layer for use in tissue lamina as well as a medical devices having a vascular layer and kits.

RELATED APPLICATIONS

This application is a continuation of PCT international application Ser.No. PCT/US2008/004872, filed Apr. 14, 2008, designating the UnitedStates and published in English on Oct. 23, 2008 as publication WO2008/127732 A3, which claims priority to U.S. Provisional ApplicationSer. No. 60/923,312, filed Apr. 12, 2007 and U.S. ProvisionalApplication Ser. No. 60/923,474, filed Apr. 12, 2007. The entirecontents of the aforementioned patent applications are incorporatedherein by this reference.

STATEMENT OF GOVERNMENT SUPPORT

Some of the work described herein was sponsored by the NationalInstitutes of Health, Grant Nos. 1 F32 DK076349-01 and T32DK07754-09.The U.S. Government has certain rights in this invention.

JOINT RESEARCH AGREEMENT

The presently claimed invention was made by or on behalf of the belowlisted parties to a joint research agreement. The joint researchagreement was in effect on or before the date the claimed invention wasmade and the claimed invention was made as a result of activitiesundertaken within the scope of the joint research agreement. The partiesto the joint research agreement are 1) The General Hospital Corporationand 2) The Charles Stark Draper Laboratory, Inc.

BACKGROUND OF THE INVENTION

Although organ transplantation has emerged as viable treatment forpatients with end stage organ disease, there is a uniform organ shortagein the United States and worldwide. Patients awaiting liver, lung andheart transplants often die before they receive an organ due to the longtransplant waiting times. Artificial organs could be used to assist oreven replace organs as a solution to the organ shortage.

Development of a tissue engineered solid organ such as a liver or kidneyis typically dependent on two main components—the parenchymal cells anda vascular network to supply oxygen and nutrients to the parenchymalcells. The diffusion distance of oxygen and nutrients from a bloodvessel through tissue is very short (e.g., a few hundred microns). Ifcells, such as hepatocytes are grown in a three-dimensional containerand placed in the body near a blood vessel, only the cells in closeproximity to the blood vessel will survive. Over time, new blood vesselsmay grow into the implanted cells, however, many of the cells that arefar from the existing blood vessels will die without immediate bloodsupply.

Present designs provide a vascular network as a central part of thescaffold for a tissue engineered solid organ. The vascular networkserves as the blood supply to deliver oxygen and nutrients to the othercells which are also placed in the scaffold to give the organ itsfunction (e.g., hepatocytes for a tissue engineered liver). Thisapproach allows a vascular network to be designed for the particularorgan from the inlet vessels which are anastomosed to the nativecirculation to the smallest vessels which perfuse the parenchymal cells.This tissue engineered organ is implanted with blood vessels alreadyadequately located in proximity to the parenchymal cells. This allows athick, solid organ such as the liver, lung, heart, kidney or otherorgans or tissues to be created and implanted.

In the body, blood vessels which supply organs typically enter theorgans as one single vessel (typically an artery) and then branch in apattern, reducing their diameter and greatly increasing their surfacearea until they form the smallest vessels known as capillaries. Thecapillaries supply the cells of the organ with oxygen and nutrients andremove waste products. From the capillaries, the vessels coalesce in asimilar branching pattern to exit the organ often as a single vessel(typically a vein). There is a need in the art for tissue engineeredorgans having such a physiological vasculature network to providesustained function following implantation.

SUMMARY OF THE INVENTION

It is an object of the subject technology to provide a tissue engineeredorgan which has a structure similar to natural organs and is capable ofsimilar performance for sufficient periods of time without malfunction.Preferably, the tissue engineered organ will have low thrombogenicityand a high packing efficiency.

It is envisioned that the subject technology may be used to replace anorgan, in vivo or ex vivo, assist an organ, temporarily replace an organand ascertain the efficacy and safety of a drug on human cells.

One aspect of the subject technology provides a method of fabricating ascaffold comprising a fluidic network. The method includes the steps of:(a) generating an initial vascular layer for enclosing the chamber andproviding fluid to the cells, the initial vascular layer having anetwork of channels for fluid; (b) translating the initial vascularlayer into a model for fluid dynamics analysis; (c) analyzing theinitial vascular layer based on desired parameters selected from thegroup consisting of a characteristic of a specific fluid, an inputpressure, an output pressure, an overall flow rate and combinationsthereof to determine sheer stress and velocity within the network ofchannels; (d) measuring the sheer stress and the velocity and comparingthe obtained values to predetermined values; (e) determining if eitherof the shear stress or the velocity are greater than or less than thepredetermined values, and (f) optionally modifying the initial vascularlayer and repeating steps (b)-(e). The initial vascular layer may befabricated from collagen.

Another aspect of the subject technology is directed to a compositioncomprising a vascular layer for use in a tissue lamina. The vascularlayer includes a substrate defining a network of channels having atleast one input channel and at least one output channel and at least twointermediate channels at least partially connecting the at least oneinput channel and the at least one output channel, each channel having aheight and a width, wherein the intermediate channels are formed inaccordance with Murray's law by varying said height and width withrespect to adjacent portions of the input and output channels.

The subject technology also provides an artificial vascular networkincluding a substrate defining a network of channels having at least oneinput channel and at least one output channel and at least twointermediate channels at least partially connecting the at least oneinput channel and the at least one output channel. Each channel has aheight and a width, wherein the intermediate channels are formed inaccordance with Murray's law by varying said height and width withrespect to adjacent portions of the input and output channels. Theartificial vascular network is prepared by a process including the stepsof: (a) fabricating a substrate defining a network of channels, whereinthe channels provide fluid to cells; (b) translating the network ofchannels into a model for fluid dynamics analysis; (c) analyzing thenetwork of channels based on desired parameters selected from the groupconsisting of a characteristic of a specific fluid, an input pressure,an output pressure, an overall flow rate and combinations thereof todetermine sheer stress and velocity within the network of channels; (d)measuring the sheer stress and the velocity and comparing the obtainedvalues to predetermined values; and (e) determining if either of theshear stress or the velocity are greater than or less than thepredetermined values.

Another aspect is a medical device for assisting or replacing an organincluding a header layer having a nozzle to connect to a vessel anddefining a distribution network in fluid communication with the nozzle,and a first vascular layer having a substrate defining a vascularnetwork of channels in fluid communication with the distributionnetwork, the vascular network including at least one input channel thatbifurcates repeatedly into portions, which rejoin to form an outputchannel, the input and output channels having a height and a width,wherein the bifurcated portions are formed in accordance with Murray'slaw by varying said height and width with respect to adjacent portionsof the input and output channels. Another layer defines a chamber forholding parenchymal cells that is configured to receive oxygen andnutrients from a fluid in the vascular layer.

A membrane separates the vascular layer from the parenchymal chamber.Preferably, the membrane is semi-permeable and the pore size of themembrane is smaller than the cell diameters, thus, cells will not beable to pass through (i.e., a low permeability for animal cells), whilelow molecular weight nutrients, gases and fluids can pass through (i.e.,a high permeability for small compounds), thereby providing adequatecell-to-cell signaling. Cell sizes vary but in general, the cell sizesare in the range of microns. For example, a red blood cell has adiameter of about 8 μm. Preferably, the average membrane pore size is ona submicron-scale to ensure effective screening of the cells. For lungapplication and the like, the membrane should also allow passage ofcarbon dioxide, oxygen and like gases therethrough.

A vascular network for creating tissue engineered organs in accordancewith the subject technology follows the same approach as the bloodvessels within a natural organ. The replication of physiological designprinciples in vascular networks is referred to herein by the phrase“biomimetic vascular networks”.

The subject technology described herein includes the theory, concepts,design, manufacturing, testing and applications of biomimetic vascularnetworks. These vascular networks have primary application as a centralpart of a scaffold to create a tissue engineered structure such as anorgan or other mammalian tissue. There are additional applications ofthis technology, for example, as a tool, e.g., a platform for drugdiscovery, development and/or evaluation (e.g., toxicity, safety and/orefficacy) and as a platform for in vitro or in vivo research andtesting.

It should be appreciated that the present invention can be implementedand utilized in numerous ways, including without limitation as aprocess, an apparatus, a system, a device, a kit (e.g., a kit comprisingone of the platforms described herein and instructions for use), amethod for applications now known and later developed or a computerreadable medium. These and other unique features of the system disclosedherein will become more readily apparent from the following descriptionand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and many of the attendant advantages of this inventionwill readily be appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 illustrates a perspective view of a bifurcating portion of avascular network in accordance with the subject technology;

FIG. 2 illustrates a perspective view of a trifurcating portion of avascular network in accordance with the subject technology;

FIG. 3 is a top view of a vascular network design in accordance with thesubject technology;

FIG. 4 is a more detailed top view of a portion of the vascular networkdesign of FIG. 3;

FIG. 5 is an exploded view of implant components using the vascularnetwork design of FIG. 3 in accordance with the subject technology;

FIG. 6 is an assembled perspective view of the implant of FIG. 5;

FIG. 7 is an assembled cross-sectional view of an implant using a simplevascular network design in accordance with the subject technology;

FIG. 8 is cross-sectional view of another implant with multiple vascularlayers in accordance with the subject technology;

FIG. 9 is a graph of the results of the in vitro blood tests and theanalysis of blood flow through the implant of FIG. 6;

FIG. 10 a perspective view of another vascular network design inaccordance with the subject technology;

FIG. 11 is a detailed view of the vascular layer of FIG. 10;

FIG. 12 is a graph of the results of the in vitro blood tests and theanalysis of blood flow through an implant using the vascular layer ofFIG. 10;

FIG. 13 is a vascular network design which utilizes a repeatingpolygonal pattern in accordance with the subject technology;

FIG. 14 is a portion of a vascular network created with a collagen filmin accordance with the subject technology;

FIG. 15 is an implant in accordance with the subject technology in placeas a liver assist device; and

FIG. 16 is an implant in accordance with the subject technology in placeas a lung assist device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention overcomes many of the prior art challengesassociated with tissue engineered vascular networks and artificial solidorgans. The advantages, and other features of the technology disclosedherein, will become more readily apparent to those having ordinary skillin the art from the following detailed description of certainembodiments taken in conjunction with the drawings which set forthrepresentative embodiments of the present invention and wherein likereference numerals identify similar structural elements.

It is to be understood that the subject technology is not intended to belimited to the particular constructs and methods described in thedescribed embodiments, as one skilled in the art can extend the conceptsinvolved using variations which are obvious after reading the presentdisclosure. Although any methods and materials similar or equivalent tothose described herein may be useful in the practice of the subjecttechnology, certain compositions, films, methods and materials aredescribed below. All relative descriptions herein such as top, bottom,left, right, up, and down are with reference to the Figures, and notmeant in a limiting sense.

The process of creating an optimal vascular network design that performssufficiently is aided by identifying and learning from the fundamentalstructure of blood vessels in the body. The vascular network ofarteries, capillaries and veins is complex. However, the basicstructural principles can be utilized within the limits of currentlyavailable manufacturing processes. There are several fundamentalprinciples of blood vessels which have been utilized in the subjecttechnology and incorporated into the design of the biomimetic vascularnetworks described herein. The concepts and the resulting designs arealso advantageously refined using computational fluid dynamics (CFD)analysis.

Blood vessels in the body have a particular relationship between thediameter of a parent vessel and the diameters of the resulting daughtervessels known as Murray's Law. Generally, Murray's Law states that thecube of the diameter of the parent vessel is equal to the sum of thecubed diameters of all daughter vessels. For a bifurcating channel,Murray's Law is expressed as d₀ ³=d₁ ³+d₂ ³, where d₀ is the diameter ofthe parent vessel and d₁ and d₂ are the diameters of the daughtervessels. The reference of diameter in regard to a rectangular channelrefers to the hydraulic diameter of the channel as defined as hydraulicdiameter=4*(cross-sectional area of channel)/(channel perimeter).

The principle of Murray's law was described in the 1930's and has sincebeen confirmed in many studies. The principle of Murray's law is tooptimize the efficiency of mass transport of the blood and control theshear stress within the vessels or channels. Shear stress in bloodvessels within the body is tightly controlled within a physiologicrange. Shear stress for arteries is typically on the range of 15 to 70dynes/cm² and for veins the shear stress is typically on the range of1-5 dynes/cm². Vessel bifurcations have shear stress which is fairlyuniform over the entire network.

Controlling shear stress is very important in minimizing the formationof thrombus within the network and ensuring that oxygen and nutrientexchange occurs within normal physiologic parameters. The biomimeticprinciple of designing for appropriate shear stress was employed in thedesign of the vascular networks described herein to reduce or eliminatethrombosis with the networks.

A Vascular Network Design

Referring to FIG. 1, a bifurcating portion 10 of a representative simplevascular network is shown in perspective view. The network portion 10includes a substrate 11 having channels formed therein. A parent vesselor channel 12 bifurcates into two daughter vessels 14. According toMurray's law, if the daughter vessels 14 are equal, then Murray's lawsimplifies to d₀ ³=2(d₁ ³). For example, if the parent vessel 12 is 1000um in diameter than each of the daughter vessels 14 would be 794 um indiameter. The daughter vessels 14 further bifurcate into smaller vessels16. According to Murray's law, the resulting smaller vessels 18 and 20would be 630 um in diameter for this example.

The smaller vessels 16 rejoin to form a larger vessels 18, which thenjoin to form vessel 20. In one embodiment, the smaller vessels 16 aremost representative of the capillary vessels found in the body. However,the parent channels 12, 14 may participate in nutrient, gas, and wasteexchange in a similar manner as the smallest channels 16. Likewise thechannels 18, 20, which are formed from the smallest channels 16, aresomewhat analogous to capillaries or venules or veins in the body.Similarly, the channels 18, 20 may also participate in nutrient, gas andwaste exchange like the smallest channels 16.

The vessels 12, 14, 16, 18, 20 are formed from linear structures andsubstantially rectangular. In another embodiment, the vessels 12, 14,16, 18, 20 may be circular or elliptical nature. In the event that thevessels 12, 14, 16, 18, 20 are rectangular, the vessels 12, 14, 16, 18,20 may have an aspect ratio of 1:1, e.g., the width and depth are equal.In another embodiment, the vessels 12, 14, 16, 18, 20 may have an aspectratio of: 1:2; 2:1; or the like. Furthermore any aspect ratio such asthe range of 100:1 or 1:100 may be considered depending on theparticular application of the vascular network.

Referring to FIG. 2, a perspective view of a trifurcating portion 30 ofa vascular network is shown. The trifurcating portion 30 also includes asubstrate 31 that forms a parent vessel 32, which divides into two equaldaughter vessels 34 and a larger daughter vessel 36 as commonly occursin the body. In this event, Murray's law is d₀ ³=d₁ ³+d₂ ³+d₃ ³.Preferably, Murray's law is applied so as to achieve an adequatecapillary channel density so the maximum distance between channels 32,34, 36 does not exceed the maximum diffusion distance of oxygen andnutrients.

In one embodiment, the maximum distance between channels would notexceed 40 um. In another embodiment this maximum distance would notexceed 500 um. In another embodiment, this maximum distance would be inthe range of 200 um to 300 um. In another embodiment, a majority of thechannels would fall within a maximum distance and a minority of thechannels would have a distance therebetween higher than the preferredmaximum. A network needs to both follow or approximate Murray's law andthe other design principles described herein and achieve a branchingstrategy which results in achievement of the desired maximum distancebetween channels.

Generally, the bifurcation angle between the parent vessel and thedaughter vessels in the body is related to the relative diameters of thedaughter vessels. These relations are described in the literature. Inprinciple, if there are two daughter vessels 14 which are equal indiameter as shown in FIG. 1, the preferred angle 22 between an axis 24of the parent vessel 12 and the daughter vessel is approximately 45degrees.

Still referring to FIG. 2, when one or two daughter vessels 34 aresmaller than another daughter vessel 36, the preferred angle 38 betweenthe axis 40 of the parent vessel 32 and the daughter vessels 34 becomeslarger than 45 degrees and approaches 90 degrees as the daughter vessels34 become much smaller than the parent vessel 32 and other daughtervessel 36. Following these biomimetic principles of bifurcation angleshelps to achieve uniform blood flow with minimal shear disruption andhence platelet activation.

In the embodiment of FIG. 2, the smaller daughter channels 34 areorientated such that a bottom 42 of the channel 34 is not on the sameplane with respect to the bottom 44 of the parent channel 32 but therespective tops of channels 32, 34 are on the same plane. As a result,an edge 46 is created in the transition between channels 32, 34.Additionally, vertical edges 48 are created in the same transition. Theedges 46, 48 are potential area for shear stress concentrations, flowseparation, stasis or turbulence within the device.

Still referring to FIG. 2, the bottom 44 of the parent channel 32includes a radius of curvature or fillet 50 on each side. In the absenceof a radius of curvature 50, there is lower blood velocity in the cornerarea than in other portions of the channel 32. The addition of thefillets 50 creates a more uniform velocity in the channel 32.

The size, shape and position of the fillets 50 are selected usingcomputational fluid dynamics (CFD) analysis, which is a tool used toanalyze fluid flow to predict the behavior of a fluid within a definedmodel. One version of a CFD tool is the FLOWORKS® module within theSOLIDWORKS® 3D CAD software available from SolidWorks Corporation ofConcord, Mass. The vascular network designs are modeled in threedimensions and then analyzed with CFD analysis to iterate and evaluateperformance of the features with respect to selected parameters.

Modification of the design elements of the networks from knownbiomimetic principles may occur, however there are goals or criteria ofthe flow of fluid through the network which are established and used asa guideline or boundary condition. For example, target shear stress,inlet pressure, outlet pressure and resulting flow rate can be theprimary goals of the design. Other goals of the design may includeminimizing flow separation, minimizing areas of low flow velocity and/orstagnation.

In one embodiment, the boundary conditions defined for the CFD analysisof the network are a combination of inlet pressure, outlet pressure andflow rate. The analysis focuses on blood in a non-Newtonian model as thefluid flowing through the vascular network. After the CFD modulecompletes the analysis, the results are reviewed, specifically thepressure drop across the network, the flow within the network, the shearstress on all of the walls of the network, and the velocity within thechannels including the uniformity of velocity. If any of the parametersfall outside a target range, then the design is modified and theanalysis is repeated.

For example, if the shear stress within a certain area is too high, thatarea of the design is modified and the analysis run again. The iterativesequence of reviewing an analysis, determining an area of the designwhich is not optimal, changing the design and running the analysis againis repeated to optimize many of the features of the design. For example,the fillet or defined curvature of each edge for optimal flow can bevaried in response to CFD analysis. Referring again to FIG. 2, thedimensions of the fillets 50 relative to that of the channel 32 wereoptimized using CFD analysis.

Furthermore, the fillets 46, 48 and 49 in the transition between channel32 and channel 34 are each different parameters. Serial iterations ofdesign changes, CFD analysis and review of results can optimize not onlyeach fillet but the bifurcation as a unit so the flow at the convergenceof channels 32, 34 and 36 is uniform with minimal shear stress changesand, thus, minimal risk for blood clot formation. As a result of theiterative process, branching channel design can be formed with minimalflow disturbances.

Referring now to FIGS. 3 and 4, top views of a vascular network design60 are illustrated. The vascular network design 60 replicates the basicstructure of the vascular system of the liver, which is arranged in aradial hexagonal pattern. The vascular network design 60 includes asubstrate 61 forming channels 62 with hexagonal areas or liver lobules64 (best seen in FIG. 4) intermediate the channels 62. Each hexagonalarea of a liver has several (usually six) blood vessels at the peripheryof the lobule which undergo branching toward a central vein. The basicdesign for the vascular network design 60 was organized similar to aliver lobule. The vascular network design 60 has six radially spacedinlet channels 66 which undergo multiple bifurcations towards a centraldraining vein 68.

Unlike the liver lobule, which is a tightly packed structure of bloodvessels and cells, one can clearly observe that in certain points of thevascular network design 60, there is a significant distance betweenadjacent channels. A goal of the vascular network design 60 was to takea step towards the fundamental structure of a liver lobule but keep thevascular network branching pattern simple to understand the utility ofthe biomimetic design principles. Other designs may pursue a much denserpattern to achieve effective delivery of oxygen and nutrients.

Accordingly, in the planar vascular network design 60 of FIG. 3, theinlet channels 66 and outlet central vein 68 are oriented orthogonal tothe branching channels 62. The channels 62 are rectangular with anaspect ratio of 1:1 and hydraulic diameters calculated according toMurray's law. The channels 62 start with initial channels 70 in fluidcommunication with the inlet channels 66. The initial channels 70successively branch, in a bifurcating manner, into successively smallerchannels 72, 74, 76, 78. The smallest diameter channels 78 arepreferably 200 um in diameter while the initial channels 70 from theinlet are 608 um in diameter. Since the channels 62 undergo simplebifurcations, the bifurcation angles are 45 degrees for each. Similarly,the channels 62 rejoin to form successively larger channels 80, 82, 84,86, which also have bifurcation angles of 45 degrees with the largestrejoining channels 86 being in fluid communication with the outletcentral vein 68.

Generally, in the body, the smaller the blood vessel diameter, theshorter the blood vessel, with capillaries being the shortest bloodvessels. More specifically, the literature enumerated lengths of severalvessels according vessel diameter from 4 mm diameter arteries to 8 umdiameter capillaries. Using this data in a spreadsheet program, a 3^(rd)order polynomial equation was determined through best fit analysis toderive a biomimetic length for the different diameter channels in thevascular network design 60. For a blood vessel of diameter x, the lengthy was determined by the equation, y=−1E-09x3+8E-06x2+0.0259x+0.1226.

For the vascular network design 60, the smallest channels 78 were 200 umin diameter and the biomimetic length derived from the previously statedequation was used to determine a biomimetic length of 6.79 mm. Thebranches 70, 72, 74 and 76 which preceded the smallest channel 78 andthe branches 80, 82, 84, and 86 which followed, could not be constructedwith their biomimetic lengths due to the size constraints of the mold.Therefore, the biomimetic length of the smallest channel 78 was used andthe lengths of the other channels (70, 72, 74, 76, 80, 82 and 84) werescaled to 39.2% of their biomimetic length to enable the entire vascularnetwork to fit within the defined mold size of six inches in diameter.Thus, the vascular network design 60 uses proportionally smaller lengthswhile preserving length relationships between channels of differentdiameters. In another embodiment, the biomimetic lengths of all of thechannels could be scaled in an equal fashion.

The shear stress in arteries is between 15 and 70 dynes/cm². Thechannels 62 are designed in response to blood flow under CFD analysis toform curves which minimize concentrations of shear stress. Through aniterative design process using results from repeated CFD analysis, thecurvature of the vascular network design 60 was improved to minimizeconcentrations of shear stress. The vascular network design 60 also keepat least some of the input channels 70, 72, 74, 76, 78 in thephysiological shear stress range for arteries. Additionally, asdescribed above, the channels 62 may have radii of curvature or fillets,particularly at the points of bifurcation, to improve the flowcharacteristics. In short, all of the channels 62 can be evaluated andrefined using results from repeated CFD analysis to minimize thevariation of shear stress.

The venous system in the body has comparatively larger diameters andlower shear stress than the arterial system. Accordingly, the shearstress within the venous system is typically 1 to 5 dynes/cm², which islower than the shear stress in the arterial system. The vascular networkdesign 60 has lower shear stress in the output channels 80, 82, 84, 86to minimize resistance and mimic the venous shear stress values. Thereis a balance between achieving a low shear stress value to replicate thehuman veins and having too slow of blood flow such that thrombus may beinitiated. Thus, the output channels 80, 82, 84, 86 are scaled up indiameter compared to the inlet channels 70, 72, 74, 76, 78 to achieve ashear stress value of generally between 6 and 10 dynes/cm².

In other embodiments, the shear stress values for the inflow portion ofa vascular network may correspond to normal arterial shear stress valuesand the outflow portions of the network may correspond to normal venousshear stress values. The degree of scaling up of the diameters of thevenous system may be in the range of 1% to 50% of the correspondinginflow diameters. In another embodiment, the degree of venous scalingmay be in the range of 5% to 15% of the corresponding venous diameters.Although the vascular network design 60 has a branching pattern in theoutflow portion which closely replicates the branching pattern in theinflow portion, in other embodiments, the branching pattern of theinflow and outflow portions may not be similar. Furthermore, thebranching pattern between portions of the inflow areas of the networkmay be different than other areas of the inflow network. Likewise inother embodiments, the branching pattern between portions of the outflowareas of the network may be different than other areas of the outflownetwork.

Process for Creating the Vascular Network Design

Previous vascular networks for tissue engineered organ development weremanufactured utilizing molds created using the photolithography processand replica casting using silicone as described in U.S. patentapplication Ser. No. 10/187,247, filed Jun. 28, 2002 and U.S. patentapplication Ser. No. 10/983,213, filed Nov. 5, 2004. Photolithography isonly able to create a single depth of channel or multiple depths withvertical step transitions. In contrast, the vascular network design 60has many different diameters of channels and these channels may achievethe most uniform blood flow if the channels have an aspect ratio notequal to 1:1. Furthermore, a uniform depth transition between thechannels can minimize shear stress changes. Minimizing shear stresschanges and associated flow disturbances will minimize clotting withinthe resulting device. In this regard, the device can be characterized ashave low thrombogenicity.

Although using photolithography to create a mold to manufacture thevascular network design 60 is possible, a different manufacturingprocess was used, namely electrical discharge machining (EDM) ormicro-machine tool equipment available from Microlution, Inc. ofChicago, Ill. In the EDM process, an electrode in the shape of thereverse or mirror image of the vascular network design 60 is created outof graphite using traditional milling machining processes. The electrodeis then used to vaporize metal in the desired pattern to create a metalor vascular mold (not shown). The mold is preferably a positive mold,thus the channel features are ridges projecting from a base. To createthe vascular network design 60, a material is placed over the mold in amanner in which the material takes the shape of the projected networkpattern. The mold is then removed from the material or substrate 61,leaving the imprint of the vascular network design 60 in the material.

The Assembled Implant Using the Vascular Network Design

Referring now to FIG. 5, an exploded view of an implant 100 using thevascular network design 60 is shown. The implant 100 includes aperforated layer 110 and a header layer 120, which mount on top of thevascular network design 60 to substantially enclose the channels 62 asshown in FIG. 6. The vascular network design 60 has inlets 66 thataccept inflow in a direction orthogonal to the branched pattern ofchannels 62 in the vascular network design 60. In order to direct bloodflow from a blood vessel and into the inlets 66 of the vascular networkdesign 60, the header layer 120 distributes blood from a single centralinlet 122 via radial channels 124.

On the radially outward end, the radial channels 124 form an opening126, which aligns with a respective passthrough hole 112 in theperforated layer 110. The passthrough holes 112, in turn, align with theinlets 66 of the vascular network design 60 so that blood flows from thesingle central inlet 122 to the inlets 66. Blood passes through theheader layer central inlet 122, the header layer radial channels 124,the passthrough holes 112 in the perforated layer 110, into the inlets66 of the vascular network design 60, through the branched pattern ofchannels 62, and out the central opening 68 of the vascular networkdesign 60.

Referring to FIG. 7, an assembled cross-sectional view of the implant orscaffold 100 using the simplified vascular network design 60 is shown.The blood enters the scaffold 100 via an inlet nozzle 130. The flow pathof the blood is designated by arrows “a”. The blood passes through aheader layer 120, which separates and directs the blood radially outwardthrough channels 124. The blood then flows through the passthrough hole112 in the perforated layer 110 and into the inlets 66 of the vascularlayer 60. The blood is then directed through the channels 62 of thevascular network design 66, where the blood collects in a central outlet68. The blood then proceeds through the outlet nozzle 132.

Cells particular to the type of tissue which is being generated (e.g.,hepatocytes for liver) are positioned in at least one parenchymalchamber 172 defined by a cellular layer 170. The parenchymal chamber 172is separated from the vascular network channels 62 by a semi-permeablemembrane layer 110 a (see FIG. 7). The membrane can be formed from aphysiological source (e.g., derived from a living tissue), or from abiologically compatible, nondegradable material such as cellulose,PolyDiMethylSiloxane (PDMS), PolyMethylMethacrylate (PMMA),PolyEtherSulfone (PES), PolySulfone (PS), PolyCarbonate (PC), or from adegradable material such as PLGA, PolyCaproLactone (PCL) or Biorubber,but the invention is not so limited. “Parenchymal cells” include thefunctional elements of an organ, as distinguished from the framework orstroma. Parenchymal cells can include but are not limited to smooth orskeletal muscle cells, myocytes, fibroblasts, chondrocytes, adipocytes,fibromyoblasts, ectodermal cells, including ductile and skin cells,hepatocytes, kidney cells, liver cells, cardiac cells, pancreatic isletcells, cells present in the intestine, and other parenchymal cells, stemcells including adipose or bone derived mesenchymal stem cells,embryonic stem cells and induced pluripotent stem cells, osteoblasts andother cells forming bone or cartilage, and hematopoietic cells. Thecells are not limited to parenchymal cells but can be many other kindsof cells (stem or progenitor cells).

As blood flows though the vascular network layer 60, oxygen andnutrients diffuse across the membrane layer 110 a to nourish the cellswithin the chamber 172. Waste generated by the cells in the chamber 172can diffuse back across the membrane layer 110 a and into the channels62 of the vascular network layer 60. The chamber 172 may have one ormore inlets 174 where the cells can be infused, injected or otherwiseinserted into the chamber 172 of the implant 100. The flow path of thecells is designated by arrows “b”.

In the embodiment of FIGS. 5-7, there are six radially spaced channels124 on the header layer 120 that align with six inlets 66 of thevascular network design 66. Single central inlet 122 and outlet 68 alignwith the nozzles 132, 134 but other configurations may be utilized suchas two inlets and two outlets. In another embodiment, there may be fourinlets and one outlet. In another embodiment, the number of inlets maybe in the range of 1 to 120. In another embodiment, the number ofoutlets may be in the range of 1 to 120.

The implant 100 has the nozzles 130, 132 for interconnecting with theblood vessels of the body. The inlet nozzle 130 preferably permanentlymounts to the central inlet 122 of the header layer 120 and the outletnozzle 132 permanently mounts to the outlet 68 of the vascular networkdesign 60. The nozzles 130, 132 may utilize a standard artificialvascular graft, such as a PTFE graft. The vascular graft may be securedto the connectors or nozzles 130, 132 by intrinsic compression of thegraft around the respective nozzle 130, 132, such as by one or moresutures, clamps, adhesive, locking devices or any combination there of.In another embodiment, a graft may be incorporated directly into thescaffold material without attaching to a nozzle. A graft may be directlysecured to a component of the scaffold such as the header layer 120 byone or more sutures, clamps, adhesive, locking devices or anycombination there of.

Another Assembled Implant

Referring to FIG. 8, another assembled implant 200 is shown incross-sectional view. As will be appreciated by those of ordinary skillin the pertinent art, the implant 200 utilizes similar principles to theimplant 100 described above. Accordingly, like reference numeralspreceded by the numeral “2” instead of the numeral “1” are used toindicate like elements whenever possible. The primary difference of theimplant 200 in comparison to the implant 100 is having multiple vascularlayers or vascular network designs 260 a, 260 b joined to a singleheader layer 220. Each vascular network design 260 a, 260 b couples torespective semi-permeable membrane layers 210 a, 210 b and cellularlayers 270 a, 270 b.

The number of vascular layers may vary such as within the range from 2to 1000 or more. The number and size of the layers for a particularimplant depends on the particular tissue, and the amount of tissuerequired, and the size of the patient. The number of scaffolds may alsobe increased to accommodate the needs of the patient.

For multiple vascular layers 260 a, 260 b, the inlets 266 a, 266 b areredesigned to balance flow between the layers 260 a, 260 b. For example,the inlets 266 a, 266 b may taper. Alternatively, each vascular layer260 a, 260 b may be fed by only half of the radial channels 224 of theheader layer 220. As shown in FIG. 8, the vascular layer 260 a has inletholes 266 a which extend there through in a tapered manner. Thus, bloodenters both the channels 262 a, 262 b of both vascular layers 260 a, 260b by partially passing through the vascular layer 260 a into thevascular layer 262 b below.

In use, blood from the patient enters the implant 200 via the inletnozzle 230 and proceeds into the header layer 220, where the blood isdirected radially via the distribution channels 224. The blood proceedsinto through the passthrough holes 212 in the perforated layer 210 andinto the inlets 266 a, 266 b of the vascular layers 260 a, 260 b.

With more than one vascular layer 260 a, 260 b, a portion of the bloodflow continues vertically downward past the upper vascular layer 260 a,through respective openings 212 a in the adjacent semi-permeablemembrane layer 210 a and parenchymal layer 270 a and into the inlets 266b of the lower vascular layer 260 b. As noted above, to facilitate evendistribution of blood between the vascular layers 260 a, 260 b, therespective inlets 260 a, 260 b and openings 212 a may taper.Alternatively, select radial channels 224 may feed different vascularlayers 260 a, 260 b.

Within the vascular layers 260 a, 260 b, the blood is directed throughthe bifurcating channels 262 a, 262 b into central collecting outlets268 a, 268 b. The central collecting outlets 268 a, 268 b extend throughthe underlying semi-permeable membrane layers 210 a, 210 b and cellularlayers 270 a, 270 b to discharge the blood through the outlet nozzle232. The outlet nozzle 232 may direct the blood back into the bloodvessel of the patient or to another device for further processing priorto reentry. The semi-permeable membranes 210 a, 210 b are adjacent tothe vascular network layers 260 a, 260 b to separate the vascularnetwork layers 260 a, 260 b from the adjacent parenchymal layers 270 a,270 b, but still allow oxygen and nutrients to pass. The parenchymallayers 270 a, 270 b form chambers 272 a, 272 b, which contain cellscorresponding to the tissue which the implant 200 is supplementing orreplacing.

In operation, there may be more than one implant or scaffold with aplurality of vascular layer and other layers. The number of implants orscaffolds may be in the range of 1 to 50. In another embodiment thenumber implants or scaffolds may be in the range of 2 to 8. The implantsmay have multiple inlets and outlets. For example, a series of inletsmay be utilized to supply blood to five vascular layers. The implantsmay also be connected in a parallel and/or serial configuration betweenthe supply blood vessel and the return blood vessel. Bifurcated vasculargrafts may be used as necessary. The number of scaffolds and thuscellular components may be different for each patient and thus adifferent size or configuration of device may be used for each patient.The chambers 272 a, 272 b have two inlets 274 a, 274 b where the cellscan be infused along the flow path indicated by arrow b, injected orotherwise inserted into the chamber 272 a, 272 b of the implant 200.

Implant Performance

Implants 100 according to FIG. 6 have been built and tested. The moldfor the vascular layer 60 was built with the EDM process as describedpreviously. The mold for the header layer 120 was built using atraditional milling process. The perforated layer 110 was formed using astandard Petri dish as a mold with a punch process to create the holes112. The nozzles 132 were created using a traditional machining process.The implant 100 was fabricated from a polydimethylsiloxane (PDMS), asilicone like material, as the material for the header layer 120, thevascular layers 60 and the perforated layer 110. The nozzles 132 may bemade with polycarbonate or nylon. For assembly, the header layer 120,the vascular layers 60 and the perforated layer 110 were adhered to oneanother with oxygen plasma bonding.

The implants 100 were tested with anti-coagulated sheep blood flowing atvarious flow rates and the inlet pressures of the device were measured.The results were compared to the expected results of the inlet pressureaccording to CFD analysis. FIG. 9 is a graph of the results of the invitro blood tests and the analysis of blood flow through the implant 100using computational fluid dynamics. The in vitro flow results and theCFD results had good correlation, especially at the design point of a 6mmHg pressure drop across the implant 100. There was a less than 5%variation between the in vitro performance and the expected CFD data.

Another Vascular Layer

Referring to FIGS. 10 and 11, a mold 360 for fabricating anothervascular layer is shown in perspective view. The mold 360 consists ofraised features where the vascular layer consists of channels. As willbe appreciated by those of ordinary skill in the pertinent art, avascular layer fabricated from mold 360 utilizes similar principles tothe vascular layers 60, 160, 260 described above. The primary differenceof the vascular layer fabricated from mold 360 in comparison to thevascular layers 60, 160, 260 is an increased density of channels 362.

The branching network of channels 362 in the vascular layer fabricatedfrom mold 360 has two inlets 364 and two outlets 366. A main channel 368extends from each inlet 364 into daughter channels 370 through a seriesof trifurcations and ultimately ends in a bifurcation. The daughterchannels 370 then branch off into repeating capillary-like subunits 372.

Each subunit 372 is similar although significant variation may beappropriate for certain applications. Each subunit 372 has an initialinlet channel 386 followed by a series of three bifurcations to form aseries of successively smaller channels 384, 382, 380 (as best seen inFIG. 11). In one embodiment, the smallest channels 380 are 100 um acrossand 100 um deep and all of the channels 386, 384, 382, 380 have anaspect ratio of 1:1.

After the smallest channels 380, the subunits 372 coalesce intosuccessively larger channels 378, 376, 374, 388 and the flow collects intwo outlets 366. The vascular layer 360 may also have verticallyorientated channels at the area of the inlet and outlet so multiplelayers can be assembled. It is also envisioned that a resulting implant(not shown) would have a header layer, a perforated layer, nozzles,and/or vascular grafts or tubing leading to vascular grafts and thelike. The vascular grafts may further be anastomosed to a blood vesselin the body.

More Implant Performance

The vascular design layer fabricated from mold 360 of FIGS. 10 and 11was prototyped. The mold 360 created was a positive feature mold. Themold 360 was created in delrin using micromilling technology. Using themold 360, vascular layers were created using PDMS, which was poured overthe mold 360, cured and removed. A second layer (not shown) was bondedto the top of the open vascular network to create a closed vascularnetwork. The two layers were bonded together using oxygen plasmabonding. Silicone tubing was adjoined to the inlets 364 and outlets 366.An external barbed connector and T connector were used to uniformlydirect inflow into the two inlet areas and collect outflow from the twooutlet areas.

In vitro testing was performed on the vascular network usinganticoagulated sheep blood. The blood was pumped through the vascularnetwork using a syringe pump and the inlet pressure to the vascularlayer fabricated from mold 360 was recorded over a range of flows. FIG.12 is a graph of the in vitro testing with blood compared to a datapoint from CFD analysis. The CFD analysis results for the design pointof a differential pressure of 16 mmHG yielded a flow rate of 6.35 mmHgare shown on the graph.

The in-vitro testing required approximately a 20% higher inlet pressureto achieve a similar flow rate. CFD analysis is useful to predict withreasonable accuracy the inlet pressure for a given flow rate through thevascular network fabricated from mold 360. At the flow rate of 6.35mmHg, the shear stress within the vascular network is controlled withina physiologic range. The flow disturbances are minimized as a result ofrefining the bifurcation angles and fillets for the design.

The vascular networks disclosed herein demonstrate fundamental designprinciples which may be useful for generating a wide array of scaffoldsfor tissue engineering. The designs related to the general design for aliver lobule but could be applied to other organ scaffolds also. Thevascular networks may be tailored such that the pressure across thescaffold is matched with the design of the scaffold so there is adequateblood flow in the scaffold to supply oxygen and nutrients to the cells.

A Liver Application

Referring now to FIG. 13, a vascular network design 400 which utilizes arepeating polygonal pattern similar to that of a liver is shownschematically. In another embodiment, the vascular network design may bearranged in a repeating radial pattern similar to that of FIGS. 3-8.

In the liver, polygonal (e.g., hexagonal) liver lobules are arranged ina repeating pattern to achieve a high density configuration. Thevascular network design 400 includes multiple polygonal structures 402,each polygonal structure 402 having multiple vascular inputs 404arranged in a radial pattern around a central vascular output 406. Thevascular network design 400 also would include a branching network 408of channels between the vascular inputs 404 and the central vascularoutput 406. The branching vascular network design 400 may be similar tothe design illustrated in FIGS. 3-8 in other respects.

In another embodiment, the branching vascular network design may followthe design principles as outlined above yet accomplish a high density ofvascular channels. For example, the polygonal structures 402 may bepositioned adjacent to one another such that the vascular inputs 404branch into multiple channels which are part of the network of channelsfor multiple polygonal structures. The diameters of these vascularinputs may be different from vascular inputs positioned at the edge ofthe design which may only branch into channels which contribute to asingle polygonal structure. The polygonal structures may replicate in away that nine polygonal structures combine to create a single vascularlayer. In another embodiment, the number of adjacent polygonalstructures which create a vascular layer may be in the range from 1 to100.

As described above, the flow from the patient may enter the scaffoldthrough a single feature such as an inlet nozzle. The vascular networkdesign 400 may have a header system (not shown), which evenlydistributes the inlet flow from a single feature into all of thevascular inputs 404. The header system may incorporate one or moreheader layers to achieve even flow distribution and incorporate featuresto minimize thrombus formation. In a similar fashion, there may be aheader system which collects the flow from the vascular outputs 406 anddiverts it into a single outlet feature such as a nozzle.

Referring to FIG. 15, an implant 600 in accordance with the subjecttechnology is shown in place as a liver assist device. The tissueengineered implant 600 is connected between the portal vein 602 and theinferior vena cava 604 of the liver 606. In a patient with liverfailure, their portal pressure is usually 10 mmHg or higher and theinferior vena cava pressure is often 0-2 mmHg. Thus, the differentialpressure (e.g., the difference of the inlet and outlet pressures) may be8 mmHg. Thus the flow through the scaffold for a tissue engineered livermust be adequate with approximately low pressures.

In another embodiment, a vascularized scaffold for a tissue engineeredliver may incorporate portal venous blood inflow and arterial bloodinflow into the scaffold and may have a common venous outflow. Thescaffold may be analogous to the human liver which has both portalvenous and hepatic artery inputs. The hepatic artery primarily supportsthe biliary cells within the liver and hepatocytes in the outer portionof the hepatic lobule. There may be arterial inputs within a vascularlayer in the same region as the portal venous inputs. The resultingbranching network from the arterial inputs may not extend to and connectdirectly with a central output. The arterial network may connectdirectly with the branching network from the portal vein.

In still another embodiment, the arterial vascular network would gothrough sufficient branching and have sufficient pressure drop as to notsignificantly change the pressure within the portal venous channels whenconnected thereto. The branching arterial network may be a differentlayer, or a different plane or otherwise remote from the branchingportal vein network. The branching arterial network may also connectdirectly with a central draining output which may also include outputfrom the portal venous network.

In another embodiment, if a vascularized scaffold is created for atissue engineered lung, the vascularized scaffold may be positionedbetween the pulmonary artery and the left atrium. Depending on thedegree of pulmonary hypertension of a patient with end stage lungdisease, the mean differential pressure between the pulmonary artery andleft atrium may be 12 to greater than 20 mmHg. A vascularized lungscaffold may need to have sufficient flow for a differential pressure inthis range. In another embodiment, the vascularized scaffold may betailored to create at least a portion of a tissue engineered kidney,pancreas, skeletal muscle, heart, intestine, bladder, tongue or softtissue.

A Lung Application

Referring to FIG. 16, an implant 700 in accordance with the subjecttechnology is shown in place as a lung assist device. The tissueengineered implant 700 is connected between the pulmonary artery 702 andthe left atrium of the heart 704. In this position, the tissueengineered lung implant 700 augments the function of the lung 706. Anexternal pack 708 provides oxygen, air or another gas or gas mixture tothe implant 700. In use, blood flows through the implant 700 includingthe vascular layer(s) as described in FIG. 6. The oxygen or other gasflows through the parenchymal layer, separated from the vascular layerby a gas permeable membrane. Oxygen diffuses from the parenchymal layeracross the membrane and into the blood. Likewise carbon dioxide diffusesfrom the blood across the membrane and into the parenchymal layer. Theflow of gas through the parenchymal layer washes the carbon dioxide outof the parenchymal layer. This exchange of oxygen and carbon dioxidebetween the blood and parenchymal layer performs the fundamentalfunctions of a lung, which is the oxygenation of blood and removal ofcarbon dioxide from the blood. For the tissue engineered lungapplication, the semi-permeable membrane 110 a of FIG. 7 is permeable tooxygen and carbon dioxide. The membrane can be porous, non-porous or acombination of porous and non-porous portions. For example, the membranemay be a porous material such as polycarbonate covered with a very thinnon-porous but gas permeable material such as silicone. In anotherembodiment, the membrane may be a resorbable material such as collagen.

Furthermore, the membrane may be covered with cells. In one embodimentthe portion of the membrane adjacent to the vascular layer may becovered with endothelial cells. In another embodiment, these endothelialcells may be lung endothelial cells or non-lung endothelial cells whichexpress carbonic anhydrase on their membranes. Carbonic anhydrase is anenzyme which converts bicarbonate into carbon dioxide. In the blood mostof the carbon dioxide occurs in the form of bicarbonate. In the lung,the bicarbonate is quickly converted to carbon dioxide by carbonicanhydrase and then can diffuse into the air spaces of the lung, thealveoli. In another embodiment, the membrane can be covered withendothelial cells as described above on the side adjacent the vascularlayer and covered with lung epithelial cells (Type I, Type II or both)on the parenchymal side of the membrane. For the tissue engineered lungapplication, the total surface area of the vascular layer andparenchymal layer interface is sufficient to have exchange of oxygen,carbon dioxide or both to augment the lung function of a patient.

In one embodiment, the scaffold may be placed within the body aspreviously described. In another embodiment, the scaffold may be placedoutside the body with vascular or other connections (e.g., biliary,respiratory and the like) interfacing with the scaffold by protrudingthrough the skin or another opening in the body. An externally placedscaffold may be useful for temporary support of an organ such as theliver or lung. In another embodiment, a pump may be may be positioned inthe circuit between the patient and an external scaffold to augment theflow of blood through the scaffold. Types of pumps might include aroller pump, centrifugal pump or piston pump. For example, a temporarylung assist device may interface with a patient in a such a way that twoseparate venous catheters supply the blood to the scaffold and returnblood from the scaffold. Due to inadequate pressure drop between twoveins, an external pump may be added to effect blood flow through thescaffold.

The material for the vascular network may be composed of a materialwhich allows the attachment of cells. In another embodiment, thematerial may allow the attachment of vascular cells such as endothelialcells, smooth muscle cells, and fibroblasts. In another embodiment, thematerial may be a non-resorbable material, a resorbable material or acombination of non-resorbable and resorbable materials. In anotherembodiment the material may be a combination of resorbable materials ora combination of non-resorbable materials. A representative, but notexhaustive, list of resorbable materials or biodegradable polymers forconstruction of vascular scaffolds is shown in Table 1.

TABLE 1 Aliphatic polyesters Bioglass Carboxymethylcellulose CelluloseChitin Citrate Collagen Copolymers of glycolide Copolymers of lactideElastin Fibrin Hydrogel Modified proteins Nylon-2 PLA/polyethylene oxidecopolymers PLA-polyethylene oxide (PELA) Poly (amino acids) Poly(trimethylene carbonates) Poly(alklyene oxalates) Poly(butylenediglycolate) Poly(hydroxy butyrate) (PHB) Poly(n-vinyl pyrrolidone)Poly(ortho esters) Polyalkyl-2-cyanoacrylates PolyanhydridesPolycyanoacrylates Polydepsipeptides Polydihydropyrans Poly-dl-lactide(PDLLA) Polyesteramides Polyesters of oxalic acid Polyethylene GlycolPolyethylene Oxide Polyglycan Esters Poly(Glycerol Sebacate)Polyglycolide (PGA) Polyiminocarbonates Polylactides (PLA)Poly-l-lactide (PLLA) Polyorthoesters Poly-p-dioxanone (PDO)Polypeptides Polyphosphazenes Polysaccharides Polyurethanes (PU)Polyvinyl alcohol (PVA) Poly-β-alkanoic acids Poly-β-malic acid (PMLA)Poly-ε-caprolactone (PCL) Pseudo-Poly(Amino Acids) Starch Trimethylenecarbonate (TMC) Tyrosine based polymers Glycolide/l-lactide copolymers(PGA/PLLA) Glycolide/trimethylene carbonate copolymers (PGA/TMC)Lactide/tetramethylglycolide copolymers Lactide/trimethylene carbonatecopolymers Lactide/ε-caprolactone copolymers Lactide/σ-valerolactonecopolymers L-lactide/dl-lactide copolymers Methyl methacrylate-N-vinylpyrrolidone copolymers PHBA/γ-hydroxyvalerate copolymers (PHBA/HVA) Polyhydroxyalkanoate polymers (PHA) Poly-β- hydroxypropionate (PHPA)Poly-β-hydroxybutyrate (PBA) Poly-σ-valerolactone

In another embodiment, the thickness of the material of the scaffold maybe very thin. The thickness of the scaffold material used to create apatterned vascular layer may be in the range of 0.1 um to 1000 um thick.In another embodiment, the thickness of the scaffold material may be inthe range of 1 um to 10 um thick. The scaffold or supporting tissue oforgans often comprises collagen as an abundant component.

Collagen Layers

An embodiment of this construct may include collagen films as theprimary scaffold material. Collagen is the primary component of theextracellular matrix in solid organs and other tissues including bloodvessels. The strength of collagen is highlighted by the durability ofhuman tissues. For example, human veins typically have a burst pressureof several thousand mmHg yet are so thin they are partially transparent.The strength of thin collagen films may allow the creation of avascularized scaffold with minimal scaffold component mass to maximizethe amount of functional tissue present in a particular volume of thescaffold. Cells readily adhere to collagen and may adsorb and remodelthe collagen to optimize the scaffold prior to or followingimplantation.

Collagen may also be constructed in a thin film, sheet, or other porousor non-porous construct. In a embodiment, a collagen thin film, sheet orother construct will have a degree of porosity. This porosity may beuniform in size or spacing. The pores sizes may only allow smallmolecule in the range 1 to 500 kilodaltons. In another embodiment, thepores may be in the range of 1 micron to 100 microns. The porosity ofthe collagen network may be sufficient to allow the diffusion of oxygen,carbon dioxide, proteins, carbohydrates, fats, drugs, or any otherbiologically active across the collagen network. The porosity of thecollagen may be adjusted to meet the required diffusion for a particulartarget tissue such as the heart, liver or lung.

Given the porosity of the collagen, an additional membrane between thevascular channel and the cells of the tissue or organ may not berequired. The cells of the target tissue, for example, myocardial cells,could be placed directly on the vascular network. A second layer ofvascular network could be added on top of the cells so each area withcells of the target tissue may be in contact with more than one vascularnetwork layer. Successive vascular networks could be staggered in such away that the vascular network patterns are not precisely overlying oneanother. The out of alignment arrangement may allow the cells of thetissue between the vascular network layers to be closer to a respectivechannel than if the layers were precisely aligned. The collagen filmsmay be very thin and may be patterned to form the desired branchingpatterns and chambers.

Manufacturing a vascular network pattern out of collagen may be achievedin a number of methods. A collagen film, sheet or other thin structuremay be formed into a pattern consistent with a vascular network pattern,such as the networks shown above. Once a patterned network is created incollagen, a second collagen film, sheet or other thin structure which isnot patterned may be bonded to the patterned collagen network, thuscreating a sealed or closed collagen vascular network which has anetwork of channels consistent with a desired network design. Thepatterned network created in collagen may have channels, which are thedesired depth of the final channel.

In another embodiment, the patterned network created in collagen mayhave half of the desired depth of the final channel. The patternednetwork may be bonded to a similarly patterned network, where the resultis a closed collagen vascular network with the desired depth of thefinal channel. Although the closed collagen vascular network may becreated in a such a manner that the cross section of the channel isrectangular at one point in the fabrication, upon filling the vascularnetwork with a fluid, the resulting cross section of the channel may becircular, oval, elliptical or may retain a rectangular shape.

Referring to FIG. 14, a portion 502 of a vascular network 500 createdwith a collagen film, sheet or other thin structure is shown. Theportion 502 shows a vascular channel 504 with a single bifurcationdesign. The collagen material 502 has a total thickness, which may beless than the inner diameter 506 of the channel 504. For example, thethickness of the collagen material or portion 502 may be 10 um and theinner diameter 506 of the vascular channel 504 may be 100 um. Thevascular network 500 could be manufactured by creating a patternedcollagen film or sheet and bonding an unpatterned collagen film or sheetthereto. The vascular network 500 could also be manufactured by bondingtwo patterned collagen films together as described above.

The manufacturing of a patterned collagen film may accomplished bystarting with a positive mold of the desired vascular network. Such amold has ridges corresponding to the desired channels of the network. Itis a negative of the desired structure. A solution, suspension, colloidor other mixture containing collagen may be poured or otherwise placedover the mold. The collagen mixture may then be air dried or vacuumdried. The drying may occur at or above ambient temperature.

After drying, the collagen film then may be separated from the moldresulting in a patterned collagen film. In another embodiment, thecollagen mixture may then be gelled by incubation. The resulting gel maybe air dried or vacuum dried. The resulting collagen film may be removedfrom the mold and retain the pattern and features of the mold. Apatterned collagen film may be bonded to a flat or other patternedcollagen film using adhesive, dehydrothermal cross-linking adjacentcollagen sheets, chemically cross-linking adjacent collagen sheets orphotochemically cross-linking collagen with a photosensitizing dye. Inthe embodiment where a patterned film is placed adjacent to anunpatterned or flat collagen film, the patterned film or the unpatternedmay be dry, partially dry or hydrated. A degree of hydration of one ormore of the films may result in a desirable degree of coaptation of thenon-channeled portion of the patterned film to the corresponding flatfilm.

Thin films (e.g., having thicknesses of less than 10 um) that arepatterned or unpatterned may have peripheral support elements to aid inhandling. These elements may assist in the handling of thin films duringpatterning, coaptation, bonding, stacking and attachment of othercomponents in the manufacturing process. The peripheral support elementsmay include a mesh material, a filament, a plurality of filaments, asubstantially solid material or a solid material. The support elementsmay be temporary and removed prior to implantation of the scaffoldwithin the body of a living being. The support elements may beresorbable or non-resorbable and may be implanted into the body of aliving being within the scaffold. The inlet tubes and header componentsmay also be composed of collagen materials including collagen tubes.

The collagen components may be formed of collagen from bovine, porcine,equine, ovine, human or other mammalian sources. Any collagen type orany combinations of collagen types may be used. In another embodiment,the patterned or unpatterned films may be composed of a mixture ofcollagen and non-collagen components. In another embodiment, the filmsmay be a mixture of collagen types (e.g., types I, III, IV and VII). Inanother embodiment, the non-collagen components may be materials derivedfrom natural membranes or one or more extracellular matrix proteins orany other naturally occurring portions or components of theextracellular tissues in a living being. These extracellular matrixproteins or components may be one or more of the following; fibrin,elastin, fibronectin, laminin, hyaluronic acid, heparin sulfate orchondrotian sulfate. In one embodiment, a patterned sheet which isbonded to an unpatterned sheet may be of different material compositionthan the unpatterned sheet. In another embodiment, the material for thepatterned and flat films is a combination of collagen type I, collagentype IV, fibrin and fibronectin.

In another embodiment, scaffold in accordance with the descriptionsabove may be used to ascertain the efficacy of drugs on human cells. Forexample, a scaffold may be fabricated to determine metabolism of a testagent in a certain kind of tissue. The scaffold could incubate the testagent and an enzyme, and form an enzyme-substrate complex between theenzyme and the test agent. As a result, one could detect one or moremetabolites of the test agent. For additional example, see U.S. patentapplication Ser. No. 10/215,600 filed on Aug. 9, 2002 and Ser. No.11/183,115 filed on Jul. 15, 2005.

INCORPORATION BY REFERENCE

All patents, published patent applications and other referencesdisclosed herein are hereby expressly incorporated in their entiretiesby reference. It should be understood that the foregoing disclosure anddescription of the present invention are illustrative and explanatorythereof and various changes in the size, shape, and materialcomposition, as well as in the description of the embodiments, may bemade without departing from the spirit of the invention as defined bythe appended claims.

What is claimed is:
 1. An artificial vascular layer comprising: asubstrate defining a network of channels arranged in a pattern having atleast one input channel, at least one output channel, and a plurality ofintermediate channels connecting the at least one input channel and theat least one output channel, each channel having a height, a length, awidth, and a diameter, wherein the intermediate channels undergomultiple branching and rejoining to form hexagonal areas intermediate tothe intermediate channels, wherein the intermediate channels are formedby varying said height and width with respect to adjacent portions ofthe channels and the length of a smallest of the intermediate channelsis formed based on the respective diameter whereby said smallestintermediate channel has a biomimetic length.
 2. An artificial vascularlayer as recited in claim 1, wherein the channels are rectangular withan aspect ratio of 1:1.
 3. An artificial vascular layer as recited inclaim 1, wherein the intermediate channels successively branch andrejoin, in a bifurcating manner, into successively smaller and largerportions, respectively, with bifurcation angles of 45 degrees.
 4. Anartificial vascular layer as recited in claim 1, wherein a smallestdiameter intermediate channels is preferably 200 um in diameter while alargest initial intermediate channel from the input channels are 608 umin diameter.
 5. An artificial vascular layer as recited in claim 1,wherein the biomimetic lengths of all of the channels are scaled in anequal fashion.
 6. An artificial vascular layer as recited in claim 1,wherein the intermediate channels have radii of curvature to improve theflow characteristics.
 7. An artificial vascular layer as recited inclaim 1, wherein the substrate is selected from the group consisting ofa collagen substrate, elastin in a collagen substrate, fibronectin in acollagen substrate, a silicone substrate, a silicone substrate with acollagen coating, and a silicone substrate with a heparin coating.
 8. Anartificial vascular layer as recited in claim 7, further comprisingfillets in at least the intermediate channels.
 9. An artificial vascularlayer as recited in claim 1, wherein the channels carry a fluid and thefluid is blood, plasma or media.
 10. An artificial vascular layer asrecited in claim 1, wherein the layer comprises a medical device forassisting or replacing an organ.
 11. An artificial vascular layer asrecited in claim 1, wherein the layer includes a pharmaceutical fluid sothat the layer acts as a tool for drug discovery, development and/orevaluation.
 12. An artificial vascular layer as recited in claim 1,wherein the layer comprises a platform for in vitro or ex vivo researchand testing.
 13. An artificial vascular layer as recited in claim 1,further comprising a second header layer having a nozzle in fluidcommunication with the at least one output channel to connect to a vein.14. An artificial vascular layer as recited in claim 1, wherein theintermediate channels trifurcate at least once.
 15. An artificialvascular layer as recited in claim 1, wherein the pattern is a radialpattern.
 16. An artificial vascular layer as recited in claim 1, whereinthe plurality of outward input channels is a plurality of radiallyoutward input channels.
 17. An artificial vascular layer comprising: asubstrate defining a network of channels arranged in a pattern having atleast one central output channel, a plurality of outward input channels,and a plurality of intermediate channels connecting the input channelsand the at least one output channel, each channel having a height, alength, a width, and a diameter, wherein the intermediate channelsundergo multiple branching and rejoining to form hexagonal areasintermediate to the intermediate channels, the intermediate channelsbeing formed by varying said height and width with respect to adjacentportions of the channels and the length of a smallest diameter of theintermediate channels is formed based on the respective diameter wherebysaid smallest diameter channel has a biomimetic length.
 18. Anartificial vascular layer comprising: a substrate defining a network ofchannels arranged in a pattern having at least one central outputchannel, a plurality of outward input channels, and a plurality ofintermediate channels connecting the input channels and the at least oneoutput channel, each channel having a height, a length, a width, and adiameter, wherein the intermediate channels undergo multiple branchingand rejoining to form hexagonal areas intermediate to the intermediatechannels, the intermediate channels being formed by varying said heightand width with respect to adjacent portions of the channels and thelength of a smallest of the intermediate channels is formed based on therespective diameter whereby said smallest channel has a biomimeticlength.
 19. An artificial vascular layer as recited in claim 1, whereinthe pattern is a radial pattern.